Evolutionary and future wireless systems are expected to carry significantly higher levels of traffic than today's wireless networks. Consequently, there is a significant desire to increase the aggregate capacities of both the uplink and downlink channels as much as possible. One technique that has been proposed for increasing uplink capacity is synchronous code division multiple access (S-CDMA). In this form of CDMA, all active users transmit data with synchronized, variable-length orthogonal spreading codes. This essentially removes intra-cell interference, thereby allowing users to transmit with less power while still achieving the same power versus noise requirements at the receiver. The reduction in transmission power for each user results in less overall interference.
In S-CDMA, there are a limited number of synchronous orthogonal spreading codes available for use for uplink transmissions. Hence, in order to maximize uplink capacity, it is desirable to allocate an orthogonal code to a user only when that user actually has data to transmit. This then implies the need for users to have the capability to quickly signal the resource control mechanism when uplink transmission resources are required. A previously proposed method for accomplishing this is the System Access Channel (SACH), which is a dedicated low bit rate channel allocated to all active users, and which may be used to send short signaling messages, such as idle messages when there is no data to send and transmission request messages when there is data to send. In the S-CDMA infrastructure, the spreading codes used in these SACHs are not orthogonal to those used in the data channels, thus, the SACHs and data channels interfere with one another. Since CDMA and S-CDMA are essentially interference-limited technologies, system capacity decreases as interference increases. Accordingly, there is a need for a way to minimize the interference between the data channels and SACHs to allow increased capacity while maintaining a relatively low transmission power.